1. Field of the Invention
The present invention relates to a semiconductor ring laser gyroscope in which a rotational angular velocity of an object is detected on the basis of a beat signal which is generated due to the Sagnac effect when lights emitted from the both end surfaces of a semiconductor laser circulate through an optical fiber loop in respective opposite directions, and relates particularly to a semiconductor ring laser gyroscope suitable for downsizing.
2. Description of the Related Art
An optical fiber gyroscope which utilizes the Sagnac effect to measure a rotational angular velocity (angular velocity) of an object is widely used mainly in the aircraft and rocket industries. In a conventional optical fiber gyroscope, a light emitted from an end surface of a light source is separated by a photocoupler and introduced into an optical fiber loop so as to propagate in respective opposite directions, and an angular velocity is measured by detecting a phase fluctuation caused by the Sagnac effect between two lights circulating through the optical fiber loop in respective opposite directions.
Meanwhile, recently, an optical gyroscope is proposed in which a semiconductor laser to emit lights from both end surfaces thereof is disposed in the optical fiber loop thereby constituting a laser resonator (such a gyroscope is hereinafter referred to as a semiconductor ring laser gyroscope as appropriate) (refer to, for example, Japanese Patent Application Laid-Open No. 2007-71614).
FIG. 7 schematically shows the composition of an optical gyroscope 50 as a semiconductor ring laser gyroscope disclosed in Japanese Patent Application Laid-Open No. 2007-71614. The optical gyroscope 50 includes a semiconductor optical amplifier (SOA) 51 and an optical fiber 52 which is looped and which has its both ends connected respectively to end surfaces 51A and 51B of the semiconductor optical amplifier 51. The semiconductor optical amplifier 51 emits a clockwise laser beam CW and a counter-clockwise laser beam CCW (hereinafter referred to simply as “laser beams CW and CCW”) from the end surfaces 51A and 51B, respectively, also amplifies by stimulated emission the laser beams CW and CCW which have made a circuit through the optical fiber 52 in the respective opposite directions and returned, and then emits the amplified laser beams CW and CCW into the optical fiber 52 again.
The laser beams CW and CCW propagating through the optical fiber 52 are partly extracted therefrom and introduced into an optical fiber 54 by a photocoupler 53 and then are overlapped on each other by a photocoupler 55. The laser beams CW and CCW overlapped on each other are guided to a photodetctor 57 via an optical fiber 56. The photodetector 57 performs square-law detection of the overlapped laser beams CW and CCW and detects a beat signal generated due to a difference in oscillation frequency between the laser beams CW and CCW. The difference in oscillation frequency between the laser beams CW and CCW is caused because the semiconductor optical amplifier 51 and the optical fiber 52 in combination constitute a laser resonator. That is to say, the difference results from the fact that the lengths of the clockwise and counter-clockwise laser resonators experience an effective change due to the Sagnac effect arising from the rotation of a table 60.
The beat signal detected by the photodetector 57 is sent to a spectrum analyzer 58, and a frequency (beat frequency) fB of the beat signal is detected there. An angular velocity of a rotating body (optical fiber loop) is calculated by a detection device 59 based on the following expression showing the relation between the beat frequency fB and an angular velocity Ω:fB=(4A/nλP)Ω
where A is an area of a region enclosed by the optical fiber 52, n is a refractive index of the optical fiber 52, λ is a wavelength of the laser beams CW and CCW, and P is a path length of the laser beams CW and CCW.
The semiconductor ring laser gyroscope as described above is adapted to detect an angular velocity according to a beat frequency and therefore is essentially capable of measuring an angular velocity with high detection sensitivity. However, the present inventors, while designing to reduce the size of an optical fiber loop for the purpose of downsizing the semiconductor ring laser gyroscope, found out that it is difficult or even impossible to detect an angular velocity when the optical fiber loop has a diameter of, for example, about 100 mm (almost a palm size).
It is supposed that the semiconductor ring laser gyroscope disclosed in Japanese Patent Application Laid-Open No. 2007-71614 achieves high detection sensitivity by incorporating as a light source a semiconductor optical amplifier (SOA) adapted to obtain a laser beam having a small spectral line width (a large Q value). The semiconductor optical amplifier, however, is expensive, and so it is proposed, in order to provide a low-cost semiconductor ring laser gyroscope, to use as a light source a semiconductor laser (Fabry-Perot semiconductor) as employed, for example, in an optical pickup of an optical disk drive, which emits a laser beam having a large spectral line width but is inexpensive.
Also, a polarization preserving fiber may be used as an optical fiber in order to improve the detection sensitivity of the semiconductor ring laser gyroscope. However, a polarization preserving fiber and a coupler using such a polarization preserving fiber are expensive. Further, the polarization preserving fiber must be coupled to the light source with a high accuracy rotation positioning.
Moreover, it is difficult to detect rotation direction in the semiconductor ring laser gyroscope using an optical fiber.